1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) utilizing the electro-optical anisotropy of liquid crystal, and in particular, to an LCD for achieving a high aperture ratio and a wide viewing angle.
2. Description of the Prior Art
The LCD is advantageous in that it is small and light-weight, and has low power consumption. It is thus put into practical use in such fields as in OA and AV equipment. In particular, the active matrix type which uses a thin film transistor (TFT) for the switching element can, in principle, perform static drive of 100% duty ratio in a multiplexed manner, and is used for large screen, animation displays.
FIGS. 1 and 2 show the structure of a unit pixel for a conventional LCD; FIG. 1 is a plane view, and FIG. 2 is a cross-sectional view along the line F—F in FIG. 1. A substrate 200 made of glass, etc., is provided, and a gate electrode 201 made of Cr, etc., and a gate line 202 made of Cr, etc., are formed thereon. Gate line 202 integrally connects a row of gate electrodes 201 aligned in the same row direction. Covering them, a gate insulation film 203, made of Si3N4, etc., is formed. On gate insulation film 203, there is formed an island-like amorphous silicon (a-Si) layer 204 in a region corresponding to gate electrode 201. The a-Si layer 204 will act as an operating layer for the TFT. Amorphous silicon regions doped with impurities, are formed at both ends of a-Si layer 204 creating (N+a-Si) layers 206 to act as contact layers. Between a-Si layer 204 and (N+a-Si) layers 206, an etching stopper 205 made of Si3N4 is formed as required for structural reasons. Further, on (N+a-Si) layers 206, there are respectively disposed a source electrode 207 and a drain electrode 208 both made of material having a high-melting point, such as Al/Si, etc. In regions other than a TFT region on gate insulation film 203, a pixel electrode 210 is formed, made of Indium Tin Oxide (ITO) which is transparent and conductive. Further, a drain line 209 is also provided for integrally connecting a column of drain electrodes 208 aligned in the same column direction. Covering all of the components mentioned above, an alignment layer 211 made of a polymer film, such as polyamide, is formed. Alignment layer 211 is subjected to predetermined rubbing processing for controlling the initial orientation of liquid crystal molecules. On alignment layer 211, a liquid crystal layer 230 is formed, on which another glass substrate 220 is disposed opposing substrate 200. A common electrode 211 made of ITO is formed on the entire surface of glass substrate 220, opposing substrate 200. Common electrode 221 is covered by an alignment layer 222 made of polyamide, etc., which is subjected to rubbing processing.
Liquid crystal is a nematic phase having, for instance, positive anisotropy of dielectric constant. When it is used for an LCD, a twist nematic (TN) mode is employed in which orientation vectors of liquid crystal molecules are twisted by 90 degrees between the top and bottom substrates 200, 220. A polarizing plate (not shown) is generally provided outside each substrates 200/220 such that, in the TN mode, a polarizing axis thereof matches a rubbing direction of alignment layer 211/222 on corresponding substrate 200/220. Thus, when no voltage is applied, linearly polarized light incoming through one of the polarizing plates proceeds within liquid crystal layer 230 while revolving along the twisted orientation of the liquid crystal molecules, until it comes out from the other polarizing plate. The LCD then displays white. On the other hand, a predetermined voltage is applied between pixel electrode 210 and common electrode 221, and an electric field is formed in liquid crystal layer 230, so that liquid crystal molecules change their orientation due to their dielectric constant anisotropy such that their long axes become parallel to the electric field. As a result, the twisted orientation of the liquid crystal molecules is destroyed, and incoming linearly polarized light is thus forced to stop revolving in liquid crystal layer 230. Then, only a reduced amount of light comes out from the other polarizing plate, resulting in a gradual change of a displayed color to black. The above mode in which an LCD displays white with no voltage applied and black with voltage applied is referred to as a normally-white mode, which is mainly employed for TN cells.
Another example is a DAP (deformation of vertically aligned phases)-type LCD which uses a nematic phase having a negative anisotropy of dielectric constant for an LCD, and a vertical alignment layer for DAP orientation films 211, 222. A DAP-type LCD, which is one example of those employing electrically controlled birefringence (ECB), utilizes a difference in a refractive index between a long axis and a short axis of liquid crystal molecules, i.e., birefringence, for controlling transmission and displayed colors. For this type, a polarizing plate is formed in a crossed Nicols arrangement outside each of substrates 200, 220. When voltage is applied, linearly polarized light introduced via one of the polarizing plates is converted into elliptically polarized light via birefringence in liquid crystal layer 230. The retardation of this elliptically polarized light, i.e., the difference in phase speed between ordinary and extraordinary ray, is controlled according to the strength of electric fields generated in liquid crystal layer 230, wherein the strength of the electric fields are determined depending on a voltage applied to liquid crystal layer 23. Then, colored light as desired will come out from the other polarizing plate on the LCD at a desired transmission according to the controlled retardation amount. As described above, an LCD obtains a desired transmission or displays desired hues by controlling light revolution or birefringence in liquid crystal. This controlling is effected by applying a desired voltage to liquid crystal which is sandwiched by a pair of substrates having predetermined electrodes formed thereon. In other words, in a TN mode, the strength of transmitted light can be controlled by controlling a retardation amount by changing the orientation of liquid crystal molecules. In an ECB mode, hue separation is also achieved by controlling the strength of transmitted light, which depends on wavelength. A retardation amount depends on an angle formed by a long axis of a liquid crystal molecule and the direction of an electric field generated in the liquid crystal. However, even if this angle is primarily controlled by adjusting electric field strength, a relative retardation amount will vary depending on an angle at which an observer views the LCD, i.e., a viewing angle. As viewing angle varies, the strength or the hues of transmitted light also changes. This is a problem of view angle dependency of an LCD.